Wafer producing method and laser processing apparatus

ABSTRACT

A wafer producing apparatus detects a facet area from an upper surface of an SiC ingot, sets X and Y coordinates of plural points lying on a boundary between the facet area and a nonfacet area in an XY plane, and sets a focal point of a laser beam having a transmission wavelength to SiC inside the SiC ingot at a predetermined depth from the upper surface of the SiC ingot. The predetermined depth corresponds to the thickness of the SiC wafer to be produced. A control unit increases the energy of the laser beam and raises a position of the focal point in applying the laser beam to the facet area as compared with the energy of the laser beam and a position of the focal point in applying the laser beam to the nonfacet area, according to the X and Y coordinates.

This is a divisional application of application Ser. No. 16/563,282,filed Sep. 6, 2019, which claims the benefit of Japanese PatentApplication No. 2018-172314, filed Sep. 14, 2018.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wafer producing method for producinga silicon carbide (SiC) wafer from an SiC ingot and also relates to alaser processing apparatus for forming a separation layer inside an SiCingot.

Description of the Related Art

Various devices such as integrated circuits (ICs), large scaleintegrated circuits (LSIs), and light emitting diodes (LEDs) are formedby forming a functional layer on the front side of a wafer formed ofsilicon (Si) or sapphire (Al₂O₃) and partitioning this functional layerinto a plurality of separate regions along a plurality of divisionlines. Further, power devices or optical devices such as LEDs are formedby forming a functional layer on the front side of a wafer formed ofhexagonal single-crystal SiC and partitioning this functional layer intoa plurality of separate regions along a plurality of division lines. Thedivision lines of such a wafer having these devices are processed by aprocessing apparatus such as a cutting apparatus and a laser processingapparatus to thereby divide the wafer into a plurality of individualdevice chips respectively corresponding to the devices. The device chipsthus obtained are used in various electrical equipment such as mobilephones and personal computers.

In general, the wafer on which the devices are to be formed is producedby slicing a cylindrical ingot with a wire saw. Both sides of the wafersliced from the ingot are polished to a mirror finish (see JapanesePatent Laid-Open No. 2000-94221, for example). However, when the ingotis cut by the wire saw and both sides of each wafer are polished toobtain the product, a large proportion (70% to 80%) of the ingot isdiscarded to cause a problem of poor economy. In particular, an SiCingot has high hardness and it is therefore difficult to cut this ingotwith the wire saw. Accordingly, considerable time is required forcutting of the ingot, causing a reduction in productivity. Furthermore,since this ingot is high in unit price, there is a problem inefficiently producing a wafer in this prior art.

A technique for solving this problem has been proposed (see JapanesePatent Laid-Open No. 2016-111143, for example). This technique includesthe steps of setting the focal point of a laser beam having atransmission wavelength to hexagonal single-crystal SiC inside an SiCingot, next applying the laser beam to the SiC ingot as scanning thelaser beam on the SiC ingot to thereby form separation layers in aseparation plane previously set inside the SiC ingot, and next breakingthe SiC ingot along the separation plane where the separation layershave been formed, thus separating an SiC wafer from the SiC ingot.

SUMMARY OF THE INVENTION

However, there is a case that a facet area different in crystalstructure from a nonfacet area exists in the SiC ingot. The refractiveindex in the facet area is higher than that in the nonfacet area, andthe absorptivity of energy in the facet area is also higher than that inthe nonfacet area. Accordingly, the depth and condition of theseparation layer to be formed in the facet area may become differentfrom those of the separation layer to be formed in the nonfacet area inapplying the laser beam to the SiC ingot. As a result, there is aproblem such that a step may be formed between the separation layer inthe facet area and the separation layer in the nonfacet area. Further,to grind the wafer produced from the SiC ingot to thereby obtain adesired wafer thickness, the thickness of the wafer to be produced mustbe increased in consideration of the step between the facet area and thenonfacet area. Accordingly, the efficiency of production isinsufficient.

It is therefore an object of the present invention to provide a waferproducing method which can produce an SiC wafer from an SiC ingot in thecondition where no step is present in the separation layer between thefacet area and the nonfacet area.

It is another object of the present invention to provide a laserprocessing apparatus for use in performing this wafer producing method.

In accordance with an aspect of the present invention, there is provideda wafer producing method for producing an SiC wafer from an SiC ingothaving an upper surface and a lower surface opposite to the uppersurface, the wafer producing method including: a flat surface formingstep of grinding the upper surface of the SiC ingot to thereby form aflat surface; a coordinates setting step of detecting a facet area fromthe upper surface of the SiC ingot and also setting the X and Ycoordinates of plural points lying on a boundary between the facet areaand a nonfacet area in a condition where an X axis extends in adirection perpendicular to a direction of formation of an off angledefined as an angle of inclination of a c-plane with respect to theupper surface of the SiC ingot, and a Y axis extends in a directionperpendicular to the X axis, after performing the flat surface formingstep; a feeding step of setting a focal point of a laser beam having atransmission wavelength to SiC inside the SiC ingot at a predetermineddepth from the upper surface of the SiC ingot, the predetermined depthcorresponding to a thickness of the SiC wafer to be produced, nextapplying the laser beam from focusing means included in a laserprocessing apparatus to the SiC ingot, and relatively moving the SiCingot and the focal point in an X direction parallel to the X axis,after performing the coordinates setting step, thereby forming abelt-shaped separation layer extending in the X direction inside the SiCingot, the separation layer being composed of a modified portion whereSiC is decomposed into Si and carbon (C) and a plurality of cracksextending from the modified portion along the c-plane; an indexing stepof relatively moving the SiC ingot and the focal point in a Y directionparallel to the Y axis, after performing the feeding step, therebyforming a plurality of belt-shaped separation layers arranged side byside in the Y direction; and a separating step of separating the SiCwafer from the SiC ingot along a planar separation layer composed of theplurality of belt-shaped separation layers, after performing the feedingstep and the indexing step; in which in the feeding step, energy of thelaser beam is increased and a position of the focusing means is raisedin applying the laser beam to the facet area as compared with energy ofthe laser beam and a position of the focusing means in applying thelaser beam to the nonfacet area, according to the X and Y coordinatesset in the coordinates setting step.

In accordance with another aspect of the present invention, there isprovided a laser processing apparatus for forming a separation layerinside an SiC ingot having an upper surface and a lower surface oppositeto the upper surface, the laser processing apparatus including a holdingtable for holding the SiC ingot in a condition where the upper surfaceof the SiC ingot is oriented upward; facet area detecting meansdetecting a facet area from the upper surface of the SiC ingot held onthe holding table; coordinates setting means setting and recording X andY coordinates of plural points lying on a boundary between the facetarea and a nonfacet area in a condition where an X axis extends in adirection perpendicular to a direction of formation of an off angledefined as an angle of inclination of a c-plane with respect to theupper surface of the SiC ingot, and a Y axis extends in a directionperpendicular to the X axis; a laser beam applying unit having focusingmeans applying a laser beam to the SiC ingot in a condition where afocal point of the laser beam is set inside the SiC ingot at apredetermined depth from the upper surface of the SiC ingot, thepredetermined depth corresponding to a thickness of an SiC wafer to beproduced from the SiC ingot, the laser beam having a transmissionwavelength to SiC, thereby forming a separation layer inside the SiCingot, the separation layer being composed of a modified portion whereSiC is decomposed into Si and carbon (C) and a plurality of cracksextending from the modified portion along the c-plane; an X movingmechanism for relatively moving the holding table and the focusing meansin an X direction parallel to the X axis; a Y moving mechanism forrelatively moving the holding table and the focusing means in a Ydirection parallel to the Y axis; and a control unit increasing energyof the laser beam and raising a position of the focusing means inapplying the laser beam to the facet area as compared with energy of thelaser beam and a position of the focusing means in applying the laserbeam to the nonfacet area, according to the X and Y coordinates set bythe coordinates setting means.

According to the wafer producing method of the present invention, thedepth and condition of the planar separation layer to be formed in thefacet area can be made equal to those of the planar separation layer tobe formed in the nonfacet area. Accordingly, the SiC wafer can beproduced in the condition where no step is present in the planarseparation layer between the facet area and the nonfacet area.

According to the laser processing apparatus of the present invention,the depth and condition of the planar separation layer to be formed inthe facet area can be made equal to those of the planar separation layerto be formed in the nonfacet area in applying the laser beam to the SiCingot. Accordingly, the SiC wafer can be produced in the condition whereno step is present in the planar separation layer between the facet areaand the nonfacet area.

The above and other objects, features, and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser processing apparatus accordingto a preferred embodiment of the present invention;

FIG. 2A is an elevational view of an SiC ingot;

FIG. 2B is a plan view of the SiC ingot depicted in FIG. 2A;

FIG. 3 is a perspective view depicting a flat surface forming step in awafer producing method using the laser processing apparatus depicted inFIG. 1;

FIG. 4A is a schematic view depicting an image of the SiC ingot asdetected in a coordinates setting step in the wafer producing method;

FIG. 4B is a table depicting the X and Y coordinates of plural pointslying on the boundary between a facet area and a nonfacet area as set inthe coordinates setting step;

FIG. 5A is a perspective view depicting a feeding step in the waferproducing method;

FIG. 5B is a sectional view depicting the feeding step;

FIG. 6 is a sectional view depicting an indexing step in the waferproducing method; and

FIG. 7 is a perspective view depicting a separating step in the waferproducing method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the wafer producing method and the laserprocessing apparatus according to the present invention will now bedescribed with reference to the drawings. FIG. 1 generally depicts alaser processing apparatus 2 according to this preferred embodiment. Asdepicted in FIG. 1, the laser processing apparatus 2 includes a holdingunit 4 for holding an SiC ingot 72, facet area detecting means 6 fordetecting a facet area from the upper surface of the SiC ingot 72, andcoordinate setting means 8 for setting and recording the X and Ycoordinates of plural points lying on the boundary between the facetarea and a nonfacet area. The laser processing apparatus 2 furtherincludes a laser beam applying unit 12 having focusing means 10 forapplying a laser beam to the SiC ingot 72 in the condition where thefocal point of the laser beam is set inside the SiC ingot 72 at apredetermined depth from the upper surface of the SiC ingot 72, thepredetermined depth corresponding to the thickness of a wafer to beproduced, the laser beam having a transmission wavelength to SiC,thereby forming a separation layer inside the SiC ingot 72, theseparation layer being composed of a modified portion where SiC isdecomposed into Si and C and a plurality of cracks extending from themodified portion along a c-plane. The laser processing apparatus 2further includes an X moving mechanism 14 for relatively moving theholding unit 4 and the focusing means 10 in the X direction (feedingdirection) depicted by an arrow X in FIG. 1, a Y moving mechanism 16 forrelatively moving the holding unit 4 and the focusing means 10 in the Ydirection (indexing direction) depicted by an arrow Y in FIG. 1, and acontrol unit 18 for controlling the operation of the laser processingapparatus 2. The X direction and the Y direction are perpendicular toeach other in an XY plane. The XY plane defined by the X direction andthe Y direction is a substantially horizontal plane.

The laser processing apparatus 2 further includes a base 20, and theholding unit 4 includes an X movable plate 22 mounted on the base 20 soas to be movable in the X direction, a Y movable plate 24 mounted on theX movable plate 22 so as to be movable in the Y direction, a holdingtable 26 rotatably mounted on the upper surface of the Y movable plate24, and a motor (not depicted) for rotating the holding table 26.

The facet area detecting means 6 has an imaging unit 28 for imaging theupper surface of the SiC ingot 72 held on the holding table 26. Asdepicted in FIG. 1, an inverted L-shaped support member 30 is mounted onthe upper surface of the base 20. The support member 30 is composed of avertical portion 30 a extending upward from the upper surface of thebase 20 and a horizontal portion 30 b extending horizontally from theupper end of the vertical portion 30 a. The imaging unit 28 is mountedon the lower surface of the horizontal portion 30 b at its front endportion. Further, a display unit 32 for displaying an image obtained bythe imaging unit 28 is provided on the upper surface of the horizontalportion 30 b. Preferably, the facet area detecting means 6 has imageprocessing means for performing image processing such as binarizationprocessing to the image of the SiC ingot 72 as obtained by the imagingunit 28. Preferably, the imaging unit 28 of the facet area detectingmeans 6 serves also as an imaging unit for use in performing alignmentbefore applying a laser beam to the SiC ingot 72.

The coordinates setting means 8 are electrically connected to theimaging unit 28. According to the image of the SiC ingot 72 as obtainedby the imaging unit 28, the coordinates setting means 8 functions to setand record the X and Y coordinates of plural points lying on theboundary between the facet area and the nonfacet area in the conditionwhere an X axis extends in the direction perpendicular to a direction offormation of an off angle defined as an angle of inclination of thec-plane with respect to the upper surface of the SiC ingot 72, and a Yaxis extends in the direction perpendicular to the X axis. The directionof extension of the X axis is substantially the same as the X directiondepicted in FIG. 1, and the direction of extension of the Y axis issubstantially the same as the Y direction depicted in FIG. 1.

The focusing means or condenser 10 of the laser beam applying unit 12 ismounted on the lower surface of the horizontal portion 30 b at its frontend portion so as to be spaced from the imaging unit 28 in the Xdirection. The laser beam applying unit 12 further includes a laseroscillator (not depicted) for generating a pulsed laser beam having atransmission wavelength to SiC, an attenuator (not depicted) foradjusting the power of the pulsed laser beam generated from the laseroscillator, and focal position adjusting means (not depicted) forvertically moving the focusing means 10 to thereby adjust the verticalposition of the focal point of the pulsed laser beam. The focal positionadjusting means may be configured by a ball screw connected to thefocusing means 10 so as to extend vertically and a motor for rotatingthis ball screw. In the operation of the laser beam applying unit 12, apulsed laser beam is generated from the laser oscillator and nextadjusted in power by the attenuator. Thereafter, the pulsed laser beamis focused by the focusing means 10 and applied to the SiC ingot 72 heldon the holding table 26 of the holding unit 4.

The X moving mechanism 14 has a ball screw 34 connected to the X movableplate 22 so as to extend in the X direction and a motor 36 connected toone end of the ball screw 34 for rotating the ball screw 34.Accordingly, a rotary motion of the motor 36 is converted into a linearmotion by the ball screw 34, and this linear motion is transmitted tothe X movable plate 22, so that the X movable plate 22 can be movedrelative to the focusing means 10 in the X direction along a pair ofparallel guide rails 20 a provided on the base 20. That is, the Xmovable plate 22 is slidably mounted on the guide rails 20 a extendingin the X direction.

Similarly, the Y moving mechanism 16 has a ball screw 38 connected tothe Y movable plate 24 so as to extend in the Y direction and a motor 40connected to one end of the ball screw 38 for rotating the ball screw38. Accordingly, a rotary motion of the motor 40 is converted into alinear motion by the ball screw 38, and this linear motion istransmitted to the Y movable plate 24, so that the Y movable plate 24can be moved relative to the focusing means 10 in the Y direction alonga pair of parallel guide rails 22 a provided on the X movable plate 22.That is, the Y movable plate 24 is slidably mounted on the guide rails22 a extending in the Y direction.

The control unit 18 is electrically connected to the coordinates settingmeans 8. According to the X and Y coordinates of plural points lying onthe boundary between the facet area and the nonfacet area as set by thecoordinates setting means 8, the control unit 18 functions to increasethe energy of the laser beam and raise the position of the focusingmeans 10 in applying the laser beam to the facet area as compared withthe energy of the laser beam and the position of the focusing means 10in applying the laser beam to the nonfacet area. The control unit 18,the image processing means of the facet area detecting means 6, and thecoordinates setting means 8 may be configured by separate computers orby a common computer.

The laser processing apparatus 2 further includes a grinding unit 42 forgrinding the upper surface of the SiC ingot 72 held on the holding table26 and a separating mechanism 44 for separating the wafer from the SiCingot 72 held on the holding table 26 after forming the separation layerinside the SiC ingot 72.

The grinding unit 42 includes a casing 46 mounted on the side surface ofthe horizontal portion 30 b of the support member 30 so as to be movablein the Y direction, casing moving means 48 for moving the casing 46 inthe Y direction, an arm 50 having a base end vertically movablysupported to the casing 46 and extending in the Y direction from thebase end, arm elevating means (not depicted) for vertically moving thearm 50, and a spindle housing 52 mounted on the front end of the arm 50.

A vertically extending spindle 54 is rotatably supported to the spindlehousing 52, and a motor (not depicted) for rotating the spindle 54 isbuilt in the spindle housing 52. Referring to FIG. 3, a disk-shapedwheel mount 56 is fixed to the lower end of the spindle 54, and anannular grinding wheel 60 is fixed to the lower surface of the wheelmount 56 by bolts 58. A plurality of abrasive members 62 are fixed tothe lower surface of the grinding wheel 60 so as to be annularlyarranged at given intervals along the outer circumference of thegrinding wheel 60.

Referring back to FIG. 1, the separating mechanism 44 includes a casing64 provided near the left ends of the guide rails 20 a on the base 20 asviewed in FIG. 1, an arm 66 having a base end vertically movablysupported to the casing 64 and extending in the X direction from thebase end, and arm elevating means (not depicted) for vertically movingthe arm 66. A motor 68 is connected to the front end of the arm 66, anda suction member 70 is connected to the lower surface of the motor 68 soas to be rotatable about a vertical axis thereof. A plurality of suctionholes (not depicted) are formed on the lower surface of the suctionmember 70. These suction holes of the suction member 70 are connected tosuction means (not depicted) for producing a vacuum. Further, thesuction member 70 contains ultrasonic vibration applying means (notdepicted) for applying ultrasonic vibration to the lower surface of thesuction member 70.

FIGS. 2A and 2B depict the SiC ingot 72, which is formed of SiC. The SiCingot 72 has a substantially cylindrical shape. That is, the SiC ingot72 has a substantially circular first end surface 74, a substantiallycircular second end surface 76 opposite to the first end surface 74, asubstantially cylindrical surface 78 formed so as to connect the firstend surface 74 and the second end surface 76, a c-axis (<0001>direction) extending from the first end surface 74 to the second endsurface 76, and a c-plane ({0001} plane) perpendicular to the c-axis.

In the SiC ingot 72, the c-axis is inclined by an off angle α (e.g.,α=1, 3, or 6 degrees) with respect to a normal 80 to the first endsurface 74. The off angle α is formed between the c-plane and the firstend surface 74. The direction of formation of the off angle α (i.e., thedirection of inclination of the c-axis) is depicted by an arrow A inFIGS. 2A and 2B. Further, the cylindrical surface 78 of the SiC ingot 72is formed with a first orientation flat 82 and a second orientation flat84, which are rectangular as viewed in side elevation and function toindicate crystal orientation. The first orientation flat 82 is parallelto the direction A of formation of the off angle α, and the secondorientation flat 84 is perpendicular to the direction A of formation ofthe off angle α. As depicted in FIG. 2B, which is a plan view taken inthe direction of extension of the normal 80, the length L2 of the secondorientation flat 84 is set shorter than the length L1 of the firstorientation flat 82 (L2<L1).

The SiC ingot 72 is mainly formed of hexagonal single-crystal SiC, and afacet area 86 different in crystal structure is locally present in theSiC ingot 72 as depicted in FIG. 2B. A nonfacet area other than thefacet area 86 is denoted by reference numeral 88. That is, the crystalstructure in the facet area 86 is different from that in the nonfacetarea 88.

The preferred embodiment of the wafer producing method according to thepresent invention will now be described. In this preferred embodiment,the laser processing apparatus 2 mentioned above is used to perform thewafer producing method. First, the SiC ingot 72 is fixed to the uppersurface of the holding table 26 by using a suitable adhesive (e.g.,epoxy resin adhesive) in the condition where the first end surface 74 ofthe SiC ingot 72 is oriented upward. That is, the adhesive is interposedbetween the second end surface 76 of the SiC ingot 72 and the uppersurface of the holding table 26. As a modification, a plurality ofsuction holes may be formed on the upper surface of the holding table26, and a suction force may be applied through these suction holes tothe upper surface of the holding table 26, thereby holding the SiC ingot72 on the upper surface of the holding table 26.

After fixing the SiC ingot 72 to the upper surface of the holding table26 as mentioned above, a flat surface forming step is performed toflatten the upper surface of the SiC ingot 72 by grinding, except thecase that the upper surface of the SiC ingot 72 has already beenflattened.

In performing the flat surface forming step, the holding table 26holding the SiC ingot 72 is moved to the position below the grindingunit 42. Thereafter, as depicted in FIG. 3, the holding table 26 isrotated at a predetermined speed (e.g., 300 rpm) in a counterclockwisedirection as viewed in plan. Further, the spindle 54 is also rotated ata predetermined speed (e.g., 6000 rpm) in a counterclockwise directionas viewed in plan. Thereafter, the arm 50 is lowered by operating thearm elevating means to thereby bring the abrasive members 62 intocontact with the upper surface of the SiC ingot 72 (i.e., the first endsurface 74 of the SiC ingot 72 in this preferred embodiment).Thereafter, the arm 50 is further lowered at a predetermined feed speed(e.g., 0.1 μm/s) to thereby grind the upper surface of the SiC ingot 72.Accordingly, the upper surface of the SiC ingot 72 is flattened to sucha degree that the incidence of a laser beam on the upper surface of theSiC ingot 72 is not hindered in a separation layer forming step to beperformed later. Thus, the upper surface of the SiC ingot 72 is groundto become a flat surface.

After performing the flat surface forming step, a coordinates settingstep is performed to detect the facet area 86 from the upper surface ofthe SiC ingot 72 and set the X and Y coordinates of plural points lyingon the boundary between the facet area 86 and the nonfacet area 88 inthe condition where an X axis extends in the direction perpendicular toa direction of formation of an off angle α defined as an angle ofinclination of the c-plane with respect to the upper surface of the SiCingot 72, and a Y axis extends in the direction perpendicular to the Xaxis.

In performing the coordinates setting step, the holding table 26 holdingthe SiC ingot 72 is first moved to the position below the imaging unit28. Thereafter, the imaging unit 28 is operated to image the uppersurface of the SiC ingot 72. According to an image of the SiC ingot 72as detected by the imaging unit 28 or according to an image obtained byperforming image processing such as binarization processing to the aboveimage of the SiC ingot 72, the facet area 86 is detected. Thereafter, asdepicted in FIGS. 4A and 4B, the coordinates setting means 8 is operatedto set and record the X and Y coordinates of plural points (e.g., 24points from a point a to a point x lying on the boundary between thefacet area 86 and the nonfacet area 88 in the condition where the X axisextends in the direction perpendicular to the direction of formation ofthe off angle α and the Y axis extends in the direction perpendicular tothe X axis. Furthermore, the X and Y coordinates of plural points lyingon the outer edge of the SiC ingot 72 are also set and recorded.Thereafter, the X and Y coordinates in the facet area 86 and the X and Ycoordinates in the nonfacet area 88 are set and recorded according tothe X and Y coordinates of the plural points lying on the boundarybetween the facet area 86 and the nonfacet area 88 and according to theX and Y coordinates of the plural points lying on the outer edge of theSiC ingot 72.

After performing the coordinates setting step, a feeding step isperformed by setting the focal point of a laser beam having atransmission wavelength to SiC inside the SiC ingot 72 at apredetermined depth from the upper surface of the SiC ingot 72, thepredetermined depth corresponding to the thickness of a wafer to beproduced, next applying the laser beam from the focusing means 10 of thelaser processing apparatus 2 to the SiC ingot 72, and relatively movingthe SiC ingot 72 and the focal point in the X direction, thereby forminga belt-shaped separation layer extending in the X direction inside theSiC ingot 72, the separation layer being composed of a modified portionwhere SiC is decomposed into Si and C and a plurality of cracksextending from the modified portion along the c-plane.

In performing the feeding step, the holding table 26 holding the SiCingot 72 is first moved in the X direction and in the Y directionaccording to the image of the SiC ingot 72 as obtained by the imagingunit 28 in the coordinates setting step, thereby adjusting thepositional relation between the SiC ingot 72 and the focusing means 10in the XY plane.

Thereafter, the focusing means 10 is vertically moved by operating thefocal position adjusting means to thereby set a focal point FP (see FIG.5B) inside the SiC ingot 72 in the nonfacet area 88 at a predetermineddepth from the upper surface of the SiC ingot 72, in which thepredetermined depth corresponds to the thickness of a wafer to beproduced. Thereafter, a pulsed laser beam LB having a tranmissionwavelength to SiC is applied from the focusing means 10 to the SiC ingot72 as moving the holding table 26 at a predetermined feed speed in the Xdirection coinciding with the direction perpendicular to the direction Aof formation of the off angle α. More specifically, the pulsed laserbeam LB is initially applied to the SiC ingot 72 to thereby decomposeSiC into Si and C. Thereafter, the pulsed laser beam LB is next appliedto the SiC ingot 72 and absorbed by C previously produced. Thus, SiC isdecomposed into Si and C in a chain reaction manner with the movement ofthe holding table 26 in the X direction to thereby linearly form amodified portion 90 extending in the X direction as depicted in FIG. 6.At the same time, a plurality of cracks 92 are also formed so as toisotropically extend from the modified portion 90 along the c-plane asdepicted in FIG. 6. As a result, a belt-shaped separation layer 94composed of the modified portion 90 and the cracks 92 is formed insidethe SiC ingot 72 so as to extend in the X direction as depicted in FIG.6.

In this feeding step, according to the X and Y coordinates set in thecoordinates setting step, the control unit 18 controls the laser beamapplying unit 12 so as to increase the energy of the pulsed laser beamLB and raise the position of the focusing means 10 in applying thepulsed laser beam LB to the facet area 86 as compared with the energy ofthe pulsed laser beam LB and the position of the focusing means 10 inapplying the pulsed laser beam LB to the nonfacet area 88. Therefractive index in the facet area 86 is higher than that in thenonfacet area 88. Accordingly, by controlling the laser beam applyingunit 12 as mentioned above, the depth of the focal point FP in the facetarea 86 can be made substantially equal to the depth of the focal pointFP in the nonfacet area 88 as depicted in FIG. 5B. As a result, thedepth of the separation layer 94 to be formed in the facet area 86 canbe made substantially equal to the depth of the separation layer 94 tobe formed in the nonfacet area 88. Further, the absorptivity of energyin the facet area 86 is also higher than that in the nonfacet area 88.Accordingly, by making the energy of the pulsed laser beam LB to beapplied to the facet area 86 higher than the energy of the pulsed laserbeam LB to be applied to the nonfacet area 88, the condition of theseparation layer 94 to be formed in the facet area 86 can be made equalto the condition of the separation layer 94 to be formed in the nonfacetarea 88.

This feeding step may be performed under the following processingconditions, in which the word of “defocus” means the amount of movementof the focusing means 10 toward the lower surface 76 of the SiC ingot 72from the condition where the focal point FP of the pulsed laser beam LBis set on the upper surface 74 of the SiC ingot 72.

(Nonfacet area: refractive index=2.65)

-   -   Wavelength of the pulsed laser beam: 1064 nm    -   Average power: 7 W    -   Repetition frequency: 30 kHz    -   Pulse width: 3 ns    -   Feed speed: 165 mm/s    -   Defocus: 188 μm

Depth of the separation layer from the upper surface of the SiC ingot:500 μm

(Facet area: refractive index=2.79)

-   -   Wavelength of the pulsed laser beam: 1064 nm    -   Average power: 9.1 W    -   Repetition frequency: 30 kHz    -   Pulse width: 3 ns    -   Feed speed: 165 mm/s    -   Defocus: 179 μm

Depth of the separation layer from the upper surface of the SiC ingot:500 μm

After performing the feeding step, an indexing step is performed torelatively move the SiC ingot 72 and the focal point FP in the Ydirection, thereby forming a plurality of belt-shaped separation layers94 arranged side by side in the Y direction. In this preferredembodiment, the SiC ingot 72 is moved in the Y direction relative to thefocal point FP by a predetermined index amount Li (see FIGS. 5A and 6),and the above feeding step is repeated. As a result, another belt-shapedseparation layer 94 extending in the X direction is formed adjacent tothe previous belt-shaped separation layer 94 in the Y direction. Byrepeating the feeding step and the indexing step, a plurality of similarbelt-shaped separation layers 94 can be formed inside the SiC ingot 72at the predetermined depth over the entire upper surface of the SiCingot 72. The predetermined index amount Li is set to a value less thanthe value twice the length of each crack 92, so that the cracks 92 ofany adjacent ones of the plural belt-shaped separation layers 94arranged in the Y direction can be overlapped with each other as viewedin plan. Accordingly, the wafer can be easily separated from the SiCingot 72 in a separating step to be performed later.

After performing the feeding step and the indexing step constituting aseparation layer forming step to thereby form the plural belt-shapedseparation layers 94 inside the SiC ingot 72 at the predetermined depth,a separating step is performed to separate a wafer from the SiC ingot 72along a planar separation layer composed of the plural belt-shapedseparation layers 94. In performing the separating step, the holdingtable 26 holding the SiC ingot 72 is moved to the position below thesuction member 70 of the separating mechanism 44. Thereafter, the arm 66is lowered by operating the arm elevating means to thereby bring thelower surface of the suction member 70 into close contact with the firstend surface 74 of the SiC ingot 72 as depicted in FIG. 7. Thereafter,the suction means is operated to attract the first end surface 74 of theSiC ingot 72 to the lower surface of the suction member 70 undersuction. Thereafter, the ultrasonic vibration applying means is operatedto apply ultrasonic vibration to the lower surface of the suction member70. At the same time, the motor 68 is operated to rotate the suctionmember 70. As a result, an SiC wafer 96 can be separated from the SiCingot 72 along the planar separation layer composed of the pluralbelt-shaped separation layers 94. Thus, the SiC wafer 96 is a wafer tobe produced from the SiC ingot 72.

After separating the SiC wafer 96 from the SiC ingot 72, the flatsurface forming step may be performed to the separation surface (uppersurface) of the SiC ingot 72 (the remaining SiC ingot). Thereafter, thefeeding step, the indexing step, and the separating step may besimilarly repeated to thereby produce a plurality of similar SiC wafers96 from the SiC ingot 72. The facet area 86 is formed so as to extendfrom the upper surface of the SiC ingot 72 to the lower surface thereofand have the same shape along the thickness of the SiC ingot 72 like aJapanese Kintaro bar candy such that the same Kintaro's face appears atany axial position where the bar candy is cut. Accordingly, thecoordinates setting step must be performed only in producing the firstSiC wafer 96 from the SiC ingot 72, and it is unnecessary to perform thecoordinates setting step in producing the second and subsequent SiCwafers 96 from the remaining SiC ingot 72.

According to the above preferred embodiment, the depth and condition ofthe planar separation layer composed of the plural belt-shapedseparation layers 94 to be formed in the facet area 86 can be made equalto those of the planar separation layer to be formed in the nonfacetarea 88. Accordingly, the SiC wafer 96 can be produced in the conditionwhere no step is present in the planar separation layer between thefacet area 86 and the nonfacet area 88. That is, it is unnecessary toseparate the SiC wafer 96 with the thickness thereof increased inconsideration of the step between the facet area 86 and the nonfacetarea 88, so that the efficiency of production can be improved.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A laser processing apparatus for forming aseparation layer inside an SiC ingot having an upper surface and a lowersurface opposite to said upper surface, said laser processing apparatuscomprising: a holding table for holding said SiC ingot in a conditionwhere the upper surface of said SiC ingot is oriented upward; facet areadetecting means detecting a facet area from the upper surface of saidSiC ingot held on said holding table; coordinates setting means settingand recording X and Y coordinates of plural points lying on a boundarybetween said facet area and a nonfacet area in a condition where an Xaxis extends in a direction perpendicular to a direction of formation ofan off angle defined as an angle of inclination of a c-plane withrespect to the upper surface of said SiC ingot, and a Y axis extends ina direction perpendicular to said X axis; a laser beam applying unithaving focusing means applying a laser beam to said SiC ingot in acondition where a focal point of said laser beam is set inside said SiCingot at a predetermined depth from the upper surface of said SiC ingot,said predetermined depth corresponding to a thickness of an SiC wafer tobe produced from said SiC ingot, said laser beam having a transmissionwavelength to SiC, thereby forming a separation layer inside said SiCingot, said separation layer being composed of a modified portion whereSiC is decomposed into Si and C and a plurality of cracks extending fromsaid modified portion along said c-plane; an X moving mechanism forrelatively moving said holding table and said focusing means in an Xdirection parallel to said X axis; a Y moving mechanism for relativelymoving said holding table and said focusing means in a Y directionparallel to said Y axis; and a control unit increasing energy of saidlaser beam and raising a position of said focusing means in applyingsaid laser beam to said facet area as compared with energy of said laserbeam and a position of said focusing means in applying said laser beamto said nonfacet area, according to the X and Y coordinates set by saidcoordinates setting means.